1. Field of the Invention
This invention relates to a throwaway ball end mill and finds application chiefly in the finishing of dies of which high finished accuracy is required.
2. Description of the Related Art
In the conventional throwaway ball end mill of this kind, the cutout provided in the forward end of an end mill body a is either configured to present a flat tip-receiving surface at the bottom or, as disclosed in Japanese Kokai Utility Model Koho H6-53012, configured to present a V-shaped tip-receiving surface b (FIG. 9). Moreover, the rear end face of a tool tip c which is to contact such tip-receiving surface is also configured to present a flat or V-shaped surface corresponding to the tip receiving-surface, and the tip c inserted into the cutout is rigidly secured to the end mill body a under the fastening force of a screw element d set at right angles with the axis of said cutout.
However, in the former arrangement where the tip having a flat rear end face is secured to the end mill body formed with a flat tip-receiving surface, the alignment of the end mill body with the tip is variable according to the accuracy of the screw hole in the tip and the holder pin hole, and the accuracy of the clamp pin and/or the manner of attachment of the tip. Therefore, this arrangement is not satisfactory for the mill for use as a finishing ball end mill for jobs of which high finished accuracy is required.
Furthermore, the variation in the degree of alignment between the end mill body and the tip is also dependent on the relative positional accuracy of the set holes provided in the two elements and it is impossible to compensate for the variation in alignment at locking.
In the latter arrangement where the tip c has a V-shaped projecting rear end face, the variation in alignment or centering between the end mill body a and the tip c is dictated by the accuracy of the relative positions of the V-shaped tip receiving surface c and the mounting hole in the tip c. But high finished accuracy can hardly be expected technically in the V-shaped tip receiving surface b, and the above misalignment cannot be corrected at the time of fixing the tip c. So a ball end mill of this type cannot be reliably employed as a finishing end mill for jobs demanding high finished accuracy.
Moreover, because the tip c is installed in plane contact with the flat or V-shaped tip receiving surface b of the end mill body a, the incidence of variation in plane accuracy or in the angle of the V-shaped surface detracts from the finished accuracy of the work.